After maturation in long-haul telecommunications (Nagel, Macchesney and Walker , Croft, Ritter and Bhagavatula  and Ramaswami ), fiber-optics technology is enjoying a renaissance in the form of microstructured optical fibers (MOFs). These fibers, unlike conventional single-mode fibers, have air inclusions running along their length. These air inclusions modify the transmission properties of the fiber, providing a high degree of control over the propagation of light in the fiber and leading to numerous applications (Knight, Birks, Russell and Atkin , Broeng, Mogilevstev, Barkou and Bjarklev , Eggleton, Kerbage, Westbrook, Windeler and Hale , Monro, Belardi, Furusawa, Baggett, Broderick and Richardson , Larsen, Bjarklev, Hermann and Broeng , Windeler, Wagener and DiGiovanni  and Steel, White, deSterke, McPhedran and Botten ). Whilst MOFs have many interesting properties in their own right, such as exotic nonlinearities and dispersion profiles, a variety of novel devices may be realized by post-fabrication engineering these fibers. Through modification of the MOF itself, via tapering (Birks and Li ), the introduction of fluids into the microstructure of the fiber (microfluidics Nguyen and Wereley ) or using the MOFs in novel geometries (Nguyen, Domachuk, Eggleton, Steel, Straub, Gu and Sumetsky ), devices from photonic crystal switches (Domachuk, Nguyen and Eggleton ) to ultracompact interferometers (Grillet, Domachuk, Ta'eed, Mägi, Bolger, Eggleton, Rodd and Cooper-White ) may be fabricated using these fibers. In combination, these technologies allow for almost limitless scope in device design. In this chapter we demonstrate the post-engineering of MOFs and also demonstrate devices fabricated using these post-engineering techniques. This chapter is structured as follows. We begin in Section 2 by providing a brief history of MOFs and a review of various types of MOFs, particularly PCFs, their guiding mechanisms and uses. In Section 3 we present a novel transverse interrogation technique for MOFs, with experimental results from transversely probed PCFs, in comparison with numerical simulations of the geometry. In Section 4 we discuss tapering as applied to MOFs and PCFs, both in the transversely probed and traditional longitudinal regimes and, specifically, the creation of silica nanowires (Tong, Gattass, Ashcon, He, Lou, Shen, Maxwell and Mazur ). In Section 5 we examine microfluidic tuning of MOFs and PCFs, in a variety of device geometries. In Section 6 we present the microfluidic interferometer, which uses the post-engineering methods outlined earlier to create a novel MOF-based device of enhanced functionality. We conclude with Section 7.